I would recommend the following :-
1) Shutdown ZCS
2) Make sure all ZCS processes have indeed terminated
3) Take a good backup of /opt/zimbra
4) Upgrade to ZCS 5.0.19 first
5) Start ZCS and make sure all is okay
6) repeat steps 1-3
7) Upgrade to ZCS 6.0.2
8) Repeat step 5
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